Jonval and Girard Turbines.
water leaving with little absolute tangential velocity, having given
some 70 or 80 % of its energy to the wheel. Regulation by
throttling always reducing the efficiency considerably, the wheel
is divided by horizontal plates at G, so that in the drawing there
are three separate turbines which can be shut off in succession by
lowering the hollow cylinder F. Oil is supplied to the footstep
j through a pipe, but immersed footsteps are now superseded.
Horse-power may be found either by head or impulse formulae.
The Jonval Turbine, like the Fourneyron, is a pressure turbine;
but while the latter works best above tail water, the Jonval is
always drowned or else connected to tail water by a * suction'
tube not more than 30 ft. high, and therefore full of water. Thus
a certain head may be saved, which might be lost, through com-
pulsory position of the turbine. Fig. 711 is a vertical section,
where A is the wheel, B the guide blades, and c the shaft; and
the water flowing parallel to the shaft gives the title * parallel
flow? to this class of turbine. Regulation, formerly effected by
throttling, is now preferably obtained by closing a number of guide
passages, preserving complete admission for the remainder. In
the figure the guide passages form concentric semicircles G G in
plan, and are so bent in elevation as to meet the wheel passages
A A, which form a complete circle in plan. This arrangement
provides retiring room for the sluices F F.
The Girard Turbine was introduced to provide against the
loss of efficiency which always occurs when pressure turbines
work with fractional supply. This fault being due to the
attempted driving with a pressure for which they were not
designed, Girard widened his wheel passages towards the outlet,
and ventilated them so as never to entirely fill them with water.
The energy is then purely due to velocity, and the turbine is
an impulse machine; it has also a parallel flow and complete
admission to whatever guide passages are open. In Fig. 712,
A A are the guide blades and B the wheel. The latter is keyed
to the hollow shaft p F, which, continued upward, joins the
solid shaft G and transmits the power. The whole is hung on a
pivot bearing j carried on the fixed pillar H, and the same
arrangement appears in Fig. 711. The guide passages may be
closed by vertical shutters K K, whose rods are coupled to rollers